1. Field of the Invention
The present invention relates to an image forming apparatus including a transfer member which is adapted to sandwich with the transfer member and an image support on which a toner image is supported, so that the toner image is transferred onto the transfer material.
2. Description of Related Art
In a printing process of an electrophotographic image forming apparatus, toner images in their respective colors are transferred onto one another on a transfer belt, which is an example of the image support, (primary transfer), so that a composite toner image is formed. The composite toner image is transferred (secondary transfer) onto a transfer material at a nip (referred to below as a transfer nip) between the transfer belt and a secondary transfer roller, which is an example of the transfer member. Here, examples of the type of the transfer material include plain paper, OHP film, and heavy paper.
At the time of secondary transfer, a transfer bias voltage is applied to the secondary transfer roller. A method for controlling the transfer bias voltage as described in, for example, Japanese Patent Laid-Open Publication No. 2008-275946 (see, for example, FIGS. 2 to 5) will be described below in summary.
In the image forming apparatus, for example, constant current control is performed when the transfer material is not at the transfer nip (referred to below as during non-transfer time), such as at the time of warm-up. During the constant current control, a voltage from a power supply is applied to the secondary transfer roller. An ammeter detects current from the power supply to the secondary transfer roller (referred to below as transfer current). Moreover, a control unit continually monitors the voltage outputted by the power supply. The control unit holds a voltage value Vout of the power supply so that the value detected by the ammeter can be kept at a constant value Icc (e.g., 20 μA).
Furthermore, in the image forming apparatus, a memory unit has a plurality of first tables stored therein. For example, the first table is prepared for each type of transfer material. Moreover, each first table lists a transfer bias voltage calculation formula for each range of absolute humidity. The calculation formula is created in advance on the basis of experiments, etc.
In determining the transfer bias voltage, the control unit initially receives information concerning the type and the size (at least the width) of the transfer material to be used in the current printing process. Specifically, the information is inputted by the user manipulating an operating panel (not shown) of the image forming apparatus before pressing a print start button. The control unit receives a print command, which includes the information, from the operating panel.
Furthermore, the control unit derives absolute humidity around the secondary transfer roller by a well-known method. The control unit refers to the first table to identify a calculation formula on the basis of a combination of the type of the transfer material and the absolute humidity, and assigns, to the calculation formula, a voltage value Vout that is currently being held, thereby calculating a transfer bias voltage.
Thereafter, for example, a scanner included in the image forming apparatus reads an image of a document set by the user, and the control unit acquires image data that represents the read document image.
Furthermore, the memory unit has a plurality of second tables stored therein. For example, the second table is prepared for each combination of a type of transfer material and a coverage. Here, the coverage refers to a proportion of an area occupied by the composite toner image (referred to below as a toner area) to a printable area of the transfer material. Each second table lists at least an upper limit of transfer current for each combination of a width of the transfer material and absolute humidity. The upper limit is obtained in advance on the basis of experiments, etc.
In determining the upper limit of transfer current, the control unit initially analyzes the acquired image data to identify the coverage. The control unit identifies the second table to be used for the current secondary transfer on the basis of the combination of the type of the transfer material and the coverage, and thereafter, the control unit reads the upper limit of transfer current from the identified second table on the basis of the combination of the absolute humidity and the width of the transfer material.
As described earlier, the transfer belt supports the composite toner image thereon. Moreover, the power supply applies the transfer bias voltage to the secondary transfer roller. The composite toner image on the transfer belt is transferred onto the transfer material introduced to the transfer nip (secondary transfer). The above operation from “the constant current control during non-transfer time” to “the derivation and application of the transfer bias voltage” is the same as in a well-known active transfer voltage control (ATVC) method.
During secondary transfer, the ammeter continues to detect the transfer current. If the value detected by the ammeter exceeds the determined upper limit, the control unit performs upper limit current control, thereby gradually changing the transfer bias voltage to the secondary transfer roller. As a result, the transfer current value is kept below the upper limit.
The image forming apparatus described in Japanese Patent Laid-Open Publication No. 2008-275946 performs upper limit current control, so that toner images on the image support can be transferred onto transfer materials of various resistance values with high transfer efficiency. However, suppliers of transfer materials do not necessarily manage resistance values, and some transfer materials distributed in the global market have considerably lower resistance values than transfer materials distributed in the Japanese domestic market. Accordingly, the image forming apparatus is required to transfer a toner image onto such a transfer material with high transfer efficiency.